The sterilization of metal instruments is typically accomplished by apparatus that heats the instrument to a temperature at which the biological infecting organism is killed. This process is both costly and slow and many attempts have been made to develop processes that directly attack the organism without the overhead cost of heating the instrument. Processes, such as are disclosed in U.S. Pat. No. 5,039,495 have utilized chemical agents to attack the organisms or as disclosed in U.S. Pat. No. 5,325,020 have used physical agents such as gaseous plasmas. In connection with those processes microwaves have been used to heat the chemical agent or to induce the plasma. Other systems such as that disclosed in U.S. Pat. No. 4,861,956 have utilized aqueous environments in which water is heated to a gaseous phase which contacts and destroys the organism. In each of the systems where metal instruments are sought to be sterilized in the presence of microwave radiation the tendency of arcing to occur in the presence of sharp edges of the instruments has been a limitation of the process.
Microwave systems have been used for selectively heating the water contained in organic material. Microwaves are electromagnetic radiation having a frequency that is associated with a wavelength in the centimeter range. A preferred value is 6.25 cm. The relation between frequency and wavelength is essentially trivial: frequency=(speed of light)/wavelength.
Microwave frequencies may conveniently be chosen having frequencies that are resonantly absorbed by water molecules but not by the molecules of most plastic and ceramic materials. Thus intense microwave energy focused on an organic material may be selectively absorbed by the water contained in the material. Two benefits of microwaves are that a ceramic container for the material is not heated by the microwaves and the object is heated uniformly throughout, wherever water molecules are present. Thus one does not have to wait while heat diffuses from a heated container to the outer surface of the material to be heated and then diffuses inwardly. This has attracted many attempts to utilize microwave energy for the selective destruction of water bearing microorganisms on metal surfaces.
Metal surfaces in the presence of intense microwave radiation have a number of properties that detract from the usefulness of microwaves to sterilize such surfaces. A metal surface supports electric currents that generate intense electric and magnetic fields close to the surface. The intensity of such fields is dependent upon the radius of curvature of the metal surface, being more intense in the vicinity of a sharply curved surface and most intense in the vicinity of a discontinuity of the curvature of the surface such as would be found near a sharp metallic edge. These strong electric fields cause the local ionization of any humidity present close to the surface. The ionized humidity then supports the formation of electrical arcs resulting in an alternative path for the release of the energy from the source of microwave energy to the metal surface. Instead of the desired heating mode, the high power output of the microwave generator produces a high current density in the spark that follows the ionization path and will burn the instrument. Furthermore, once the arc is created it tends to become self sustaining and draws large currents for a relatively long period of time, sufficient to destroy the sharpness of the instrument being sterilized.
A second property of highly conductive materials is that they act as a mirror for the source of microwave radiation. Thus in the presence of a metal surface there appear virtual microwave sources as intense as the original source. These virtual sources have required the prior art to adopt shielding so as to prevent the destruction of the actual source of microwave radiation.
U.S. Pat. No. 4,861,956 assigned to Magnetronics, Inc. is for a "Microwave/Steam Sterilizer." This apparatus is a sterilizer that forms an atmosphere of gaseous water and electromagnetic energy. It is designed to operate at relatively low temperatures and for short periods of time. It tries to deal with the difficulty of using microwave energy to kill dry spores by having a water reservoir, and the tendency of microwave apparatus to self-destruct from its own energy. It comprises a waveguide conduit having a terminal aperture located one quarter wavelength from the bottom of a water reservoir and subapertures spaced one quarter wavelength from each other. The waveguide and walls of its working cavity are angled to avoid the reflection of microwave energy back to the microwave energy generator. The patent speaks of a synergy of steam and microwave energy. To accomplish this, a pool of water acts as a receptor for microwaves, and in turn provides a source of required steam and a trap to prevent the microwave energy from destroying the microwave source. The patent is concerned about the microwave energy that is reflected back to the microwave source from electrically conductive surfaces. The water is designed to act as a trap to minimize the entrance of reflected energy. A distance from the microwave outlet of one quarter wavelength from the water surface is optimal for that trapping effect.
U.S. Pat. No. 4,952,763 assigned to Snowdrift Corp. of the Netherlands Antilles is for a "System For Heating Objects With Microwaves". Two microwave frequencies are used to heat a sealed package at temperatures above 80.degree. C. placed at the geometric location where the sum field occurs. Two magnetron emitters are employed having about the same frequencies and amplitudes and radiate from opposite sides of a work space at a low frequency alternating cycle. The interference pattern from the emitters furnishes high energy concentration in parts or over all of the object to be heated. An alternative to two separate emitters is to employ a single emitter formed from one annular hollow waveguide with symmetrically disposed coupling in and coupling out locations. A pressure-tight working area can be formed by part of this hollow conductor at the coupling out location. A quarter wavelength displacement of the two coupling in points is said to generate homogeneous heating.
U.S. Pat. No. 5,039,495 assigned to Flexiclave, Inc. is for an "Apparatus For Sterilizing Articles Such As Dental Handpieces". This patent discusses prior art patent 4,400,357 which uses a liquid reaction agent and microwave radiation restricted to a narrow liquid-containing vessel while the article to be sterilized is kept outside the radiation field, in order in order to avoid the formation of spark gaps at border surfaces and seams of the article which causes surface destruction. This patent also discussed prior art patent 3,753,651, which concerns the non-thermal effects of microwave radiation and notes the improvement in sterilization that occurs in a humid environment when microwave radiation is present. The disclosure of U.S. Pat. No. 5,039,495 is itself concerned with a microwave transparent pouch into which the instruments and a sterilant solution is placed within the cavity of a microwave radiator. The patent says that surprisingly no arcing occurs despite microwave irradiation of the metal instrument. The patent suggests an alternative embodiment in which a shield is used to prevent the microwaves from reaching the metal instruments. Apparently, (Col. 9 line 8), this patent discloses that it would not be possible to simultaneously sterilize several instruments, or one with sharp edges, without having arcing occur. Therefore, the patent teaches that the instruments should be surrounded with a shield that bars the transmission of microwave radiation, such as a double layered knitted mesh of tin-copper-steel wire. It should be noted that all the claims of this patent require shielding of the instruments from the microwave radiation.
U.S. Pat. No. 3,753,651 issued to Wave Energy Systems Inc. for a "Method And Apparatus For Surface Sterilization". This uses a microwave energy field combined with a humid atmosphere of at least 50%. The inventors state that the prior art failed to find a commercially suitable use of microwaves for sterilization because it did not maintain the correct humidity level of the irradiated microorganisms. The material to be sterilized is placed into a container that is transparent to the microwaves while a moist atmosphere is confined to the interior of the container. The patent mentions the non-thermal effects of microwaves such as chemical bond breaking, accelerated diffusion of ions through membranes, and PH modification and mentions the surface-sterilization of dental tools at lower temperature than dry heat and steam sterilizers. The apparatus appears to be a conventional microwave oven with an internal structure designed to maintain the humid environment in contact with the devices to be sterilized.
U.S. Pat. No. 5,325,020 issued to AbTox, Inc. for a "Circular Waveguide Plasma Microwave Sterilizer Apparatus". This employs a microwave plasma generator for producing a gas plasma generated from a gas mixture of oxidizing and/or reducing agents. The microwaves enter a sealed gas containing chamber which is transparent to the microwave energy. It is the contact of microorganisms with the plasma that results in their destruction.
The foregoing patents require for the most part an artificial environment providing a combination of an aqueous environment with a source of microwave radiation the heat water in the environment. Those that use destructive interference of microwaves do so to avoid the effects of the reflection of microwaves from the surfaces of bulk water that is provided to form a pool to supply a relatively high humidity value in the vicinity of the apparatus to be sterilized. None of the prior art attempts to control the location of the intense radiation in the vicinity of the object to be sterilized other than by the use of electromagnet shields. None of the prior art is concerned with minimizing the electric field in the near field zone of the microwave source. Furthermore, none of the prior art attempts to utilize the thick cloud of electrons located in the near field of the magnetron to sterilize organisms residing on materials including sharp edged conductive metal surfaces.